Basin-scale modelling of soil-to-river ¹³⁷Cs transfer in the Rhône river system

Cesium-137 (¹³⁷Cs) is an anthropogenic radionuclide whose persistence in soils and sediments represents a sustained source of environmental contamination. The Rhône River basin has been affected by ¹³⁷Cs contamination from atmospheric nuclear weapons testing and the Chornobyl accident. In this study we simulate spatially the ¹³⁷Cs dynamics in the Rhône River system, including the main stem and several major tributaries. The approach builds on an existing SWAT (Soil and Water Assessment Tool) configuration of the Rhône basin and extends a radionuclide-specific mass-balance formulation previously developed at the sub-basin scale. Time-evolving soil inventories of ¹³⁷Cs from 1986 to 2016 are reconstructed using historical deposition maps. These reconstructions incorporate SWAT-derived erosion and soil turnover into a vertical migration model, providing spatially explicit initial conditions for simulations starting in 2016. Model outputs for the 2016-2020 period are evaluated against extensive observations of particulate and dissolved ¹³⁷Cs collected throughout the Rhône network. Simulated soil ¹³⁷Cs concentrations reproduce large-scale spatial patterns inherited from historical fallout and agree well with observations across most land-use classes. The modelled source term decreases annually by 2.2 %, of which 97.9 % is driven by radioactive decay. Soil erosion mobilizes 0.8 TBq yr^-1, but most of this activity is trapped within the river system. Consequently, only 0.13 ± 0.03 TBq yr^-1 (a small fraction of the total inventory) is exported at the basin outlet. This modelling framework provides a coherent basis for quantifying long-term radionuclide fluxes in large river systems affected by diffuse radioactive contamination.